Separation of cesium and strontium from nuclear waste is a pressing concern for both governments and the commercial nuclear power industry. During the first 30 years after irradiation of nuclear fuels, Cs and Sr contribute 98% of the thermal energy, and 97% of the penetrating radiation in the process waste. The most important fission-produced cesium isotope is .sup.137 CS (fission yield=6.19%), which has a half-life of 30.17 years, and decays by both .beta.-(514 keV) and .gamma.-(662 keV) emission. .sup..pi. Sr (fission yield=5.8%) has a half-life of 29.1 years and decays by .beta.-(546 keV) emission. These two elements dominate the immediate risks associated with nuclear waste, especially defense-related wastes. Separation of these elements is sometimes complicated by the high concentration of sodium in the waste, amounting to a Na/Cs mole ratio of 10.sup.5. A variety of fission products, especially transition metals, also cause problems. In order to reduce the risk to public health and prepare nuclear wastes for final disposition, selective and cost-effective separation procedures for these radionuclides must be devised. Removal of the major heat- and radiation-producing nuclides, .sup.137 CS and .sup.90 Sr, from high-level nuclear wastes also helps to reduce the cost of processing wastes for disposal, and to decontaminate wastes to low-level characteristics.
Proposed technologies for separation and recovery of Cs and Sr from nuclear waste streams often mention cobalt dicarbollide, Co(C.sub.2 B.sub.9 H.sub.11).sub.2.sup.- (CoB.sub.2.sup.-). However, application of cobalt dicarbollide to waste treatment has been deterred by the perception that highly toxic nitrobenzene is required as a diluent for the process.
Since 1976, attempts have been made to use cobalt dicarbollide in alternative solvents. The earliest report on cesium extraction using H.sup.+ CoB.sub.2.sup.- demonstrated that dilution of the nitrobenzene solvent with basic solvents such as ethyl acetate, diisopropyl ether, or tert-butanol resulted in dramatic decreases in the cesium distribution coefficients. The mechanism of this depression in extraction efficiency is believed to be the formation of H.sup.+ solvates in the organic phase, which prevents exchange of H.sup.+ ions for Cs.sup.+. By contrast, no decrease in D.sub.Cs was observed when nitrobenzene was diluted with benzene, trichloroethylene, or dichloromethane. Later reports showed that D.sub.Cs and the Cs/Rb selectivity both increased upon dilution of nitrobenzene with benzene or CCl.sub.4. Unfortunately, cobalt dicarbollide is insufficiently soluble to function in mixtures of &gt;60% nonpolar solvent in nitrobenzene and its losses into the aqueous phase strongly increase with content of nonpolar solvent. Because of the combined problems of basicity and solubility, successful CoB.sub.2.sup.- -based separations have only been achieved in exotic (expensive and having presently unknown toxicology) solvents such as nitrophenyl octyl ether and certain polyfluoroalkyl ethers. Since the extraction efficiency of cobalt dicarbollide does not directly depend on the polarity of the organic diluent, any nonbasic organic liquid should be an effective diluent for cobalt dicarbollide, provided the extractant is sufficiently soluble in that diluent. It should be noted that H.sup.+ CoB.sub.2.sup.- is insoluble in hydrocarbons.
M. Frederick Hawthorne et al. in ".pi.-Dicarbollyl Derivatives of the Transition Metals. Metallocene Analogs," J. Am. Chem. Soc. 90, 879 (1968), reported the first tetra-C-substituted cobalt dicarbollide complex, [CO((CH.sub.3).sub.2 C.sub.2 B.sub.9 H.sub.9).sub.2 ].sup.- (CoB.sub.2.sup.- Me.sub.4.sup.-). However, no attempt was made to study the extraction capabilities of this compound for Sr and Cs, and no attempt was made to study its solubility in organic solvents or lack thereof in water.
Accordingly, it is an object of the present invention to increase the solubility of derivatives of cobalt dicarbollide in nonbasic organic solvents.
Another object of the invention is to increase the solubility of derivatives of cobalt dicarbollide in nonbasic organic solvents without affecting the extraction efficiency thereof for Cs and Sr.
Yet another object of the invention is to increase the solubility of derivatives of cobalt dicarbollide in nonbasic organic solvents, while decreasing the solubility thereof in water, without affecting the extraction efficiency thereof for Cs and Sr.
Still another object another object of the present invention is to increase the solubility of derivatives of cobalt dicarbollide in nonbasic organic solvents, while decreasing the solubility thereof in water, without affecting the extraction efficiency thereof for Cs and Sr in the presence of high concentrations of sodium.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.